The invention relates to audio communication circuitry and more specifically relates to full duplex communication using a transducer that simultaneously acts as both a microphone and speaker.
The term xe2x80x9cfull duplexxe2x80x9d in the context of a communication device means that the device simultaneously transmits and receives signals. To the user, this means that he or she can simultaneously talk and listen to another party through the device. Conversely, the term xe2x80x9chalf duplexxe2x80x9d in this context means that the device can only transmit or receive at one time, but not both. Full duplex is obviously better than half duplex communication because it enables parties to communicate from remote locations as if they were standing face to face. However, full duplex communication is more difficult to implement in speaker phones because of the problem of acoustical and electrical feedback. Acoustical feedback occurs when sound from the speaker travels back to the microphone. Electrical feedback is similar, yet pertains to the electrical signals representing the audio input (the signal transmitted to the remote device) and the audio output (the signal received from the remote source). Electrical feedback occurs when the transmit and receive circuits are not completely isolated from each other and form a closed loop with a loop gain greater than one. To eliminate feedback entirely, the overall loop gain, including both acoustical and electrical effects, must be less than one.
The majority of speaker phones for hands-free and group communication are half duplex configurations that utilize fast switching circuitry to alternate between: 1) broadcasting audio output through a speaker, and 2) listening for audio input in a separate microphone. If this switching did not take place, the speaker would produce an annoying squeal due to the acoustical feedback path from the speaker to the microphone. The switching circuitry prevents the speaker and microphone from being active at the same time, and therefore, audio output from the speaker will not induce electrical signals in the microphone. While the switching avoids the squeal, it can be annoying in itself because the user cannot speak and listen at the same time. The switching circuitry compares the strength of the broadcast signals from each direction and allows only the stronger of the two to be transmitted to the opposite end.
Since full duplex communication requires a complete closed loop for simultaneously sending and receiving signals between the two locations of conversation, the overall loop gain must be less than one. One way to ensure that the loop gain is less than one is to use digital signal processing to detect feedback and attempt to cancel it. Speaker phones that employ this approach are sometimes referred to as digital full duplex speaker phones. These devices include a separate speaker and microphone, and an analog to digital conversion circuit to recognize, with adaptive filters, the signal gain variances between the transmit and receive signal paths caused by audio output of the receive signal entering the transmit portion of the loop via the microphone. In response to detecting this feedback, these devices use electronically controlled attenuators in each side of the loop to ensure that the loop gain is less than one.
While the attenuators can reduce the annoying squeal of feedback, they can tend to undermine performance of the device by reducing the gain on the audio output to such an extent that is difficult for the user to hear. At times, the attenuator needs to reduce the gain on the receive signal so much that the user cannot here the other party""s voice. In addition to this drawback, the digital configurations are several times more costly then the half duplex speaker phone configurations.
One proposed solution to the acoustical feedback problem is shown in U.S. Pat. No. 4,002,860, which describes a communication device that uses a single transducer as both a speaker and microphone to eliminate acoustical feedback. The circuit design shown in this patent does not effectively eliminate electrical feedback however. This circuit uses devices called hybrid transformers in an attempt to isolate the transmit and receive signals from each other. The telephone circuits used to provide isolation in this circuit are actually only capable of providing about 15dB of isolation. The amount of isolation is also highly dependent on the degree to which the circuit can match the impedance of the transducer and of the telephone line. Because of the circuit""s inability to isolate the transmit and receive signals, it will generate a significant amount of feedback for this reason alone.
Another drawback of the circuit shown in the ""860 patent is that the circuit applies a load across the transducer. Loading of the transducer can significantly undermine the effectiveness of this circuit because it interferes with the transmit signal induced in the transducer from an acoustical voice input. In a typical microphone, the signals induced from the user""s voice are quite small-on the order of 10 mV. Any loading of the transducer draws away the energy of the induced signal. To deal with these losses, the circuitry for processing the transmit signal can amplify the small voice signals, but if there is any feedback of the receive signal to the amplifier in the transmit circuitry, the feedback problem highlighted above becomes even worse.
The invention provides circuits that support a bidirectional signal path for simultaneous transmit and receive signals at one port while maintaining separation between the transmit and receive signals at their respective ports. In telephone terminology, a port corresponds to a pair of wires. Thus, the invention provides an interface between a two wire and a four wire configuration. This feature allows the circuits to support full duplex communication while maintaining a loop gain of less than one to address feedback problems. To accomplish this, the circuits employ amplifiers arranged in either a bilateral T hybrid or balanced impedance configuration.
While suitable for a variety of applications, one main application for the invention is in telephone devices. Within a telephone device, the invention can be used as a transducer interface and as a telephone line interconnect circuit. In a transducer interface, the invention provides a bi-directional signal interface to a transducer that acts as both a speaker/ear phone and a microphone. In a telephone line interconnect, the invention provides a bi-directional signal interface for simultaneously transmitting signals to and receiving signals from a telephone line. The transducer interface circuit drives a receive signal onto a transducer to create audio output while simultaneously generating a transmit signal from audio input to the transducer. The telephone line interconnect circuit drives a transmit signal through a transformer to send the transmit signal to a telephone line while simultaneously transferring a receive signal from the telephone line. In both applications, the circuits provide a bidirectional signal path (simultaneous transmit and receive signals) at one port and achieve separation of the transmit and receive signals being delivered to their respective ports.
One aspect of the invention is a bilateral T hybrid circuit configuration. The bilateral T hybrid includes two op amps, with the inverting terminal of one connected to the non-inverting input of the other. This node between the two op amps provides a T connect on, which carries a combined outgoing and incoming signal. One of the op amps is configured to receive a first input signal at its non-inverting input. Through the current mirror effect, this first input signal appears at the inverting input as well and represents the outgoing signal component at the T connection. A second input signal, entering the circuit at the T connection, represents the incoming signal.
The bilateral T hybrid uses a differential amplifier capable of reducing the outgoing signal and increasing the incoming signal. The first and second op amps each generate a common mode signal component corresponding to the outgoing signal. Since this signal appears as common mode, it can be canceled with the differential amplifier. A signal component corresponding to the incoming signal does not appear as common mode at the output of the amplifiers, and as such, is not canceled by the differential amplifier. In fact, the incoming signal can be applied to the op amps so that the signal components corresponding to the incoming signal are 180 degrees out of phase at the output of the op amps. Since the differential amplifier sums the components that are 180 degrees out of phase, the incoming signal is increased by the differential amplifier. In sum, the bilateral T hybrid supports a bi-directional signal flow at the T connection, yet provides an output signal with separation between the signal components corresponding to the outgoing and incoming signals.
Another aspect of the invention is a balanced impedance configuration. In one implementation, a pair of op amps are coupled together at their non-inverting inputs. These op-amps are arranged in a voltage fed-current feedback configuration. A first input signal, representing an incoming signal enters at the common node interconnecting the op amps. It appears in common mode at the output of the op amps, and thus, can be canceled by a differential amplifier. Due to the current feedback configuration, the incoming signal is transferred, virtually without loading, to a bi-directional device coupled in the feedback path of an op amp (either one or both of the op amps). A second input signal enters the circuit at the bi-directional device and is not in common mode at the output of the op amps. Thus, the output signal corresponding to the second input is not canceled in the differential amplifier.
In a telephone line interconnect, the bi-directional device can be a transformer that links the circuit to a telephone line. In this application, the transformer transfers an incoming receive signal from the telephone line to the circuit, and the circuit transfers an outgoing transmit signal through the transformer to the telephone line. In a transducer interface, the bidirectional device can be a transducer, such as an ear phone or speaker that acts as both an audio output device and an input device. In this application, the transducer generates the incoming transmit signal from audio input, and the circuit transfers an outgoing receive signal to the transducer, where it is converted to audio output.
Another aspect of the invention is the telephone line interface in the telephone line interconnect circuit. The telephone line interface includes a simulated inductor to regulate the current flow from the telephone line. The interface can also include an isolator circuit that opens the transmit signal path to the telephone line when the current in the phone line drops below a threshold or drops to zero. To prevent feedback or howl in a telephone device, the loop gain of the feedback path of the transmit and receive signals must be less than one. The isolator circuit addresses this problem by ensuring that the transmit signal path is open when the hook switch of the phone is open.
The circuits summarized above have a number of advantages. When used in a transducer interface, they support full duplex communication through a signal transducer. The circuits are effective as a transducer interface because they drive the transducer to create audio output with substantially no loading to the transducer. Since, in essence, the transducer is not loaded, it can act as a microphone to generate a transmit signal from audio input. In addition, the circuit addresses the problem of feedback by substantially canceling the receive signal component from the transmit signal path. When used in a telephone line interconnect, the circuits are useful as an interface because they provide a full duplex signal interconnect that combines the transmit with the receive signal on the telephone line while maintaining a greater separation between the transmit and receive signals than that of conventional telephone hybrid circuits or resistive-capacitive cancellation circuits.
Further advantages and features of the invention will become apparent with reference to the following detailed description and accompanying drawings.